As a method for electrically connecting semiconductor chips to a mounting board, the chips are conventionally connected to bonding pads on the board using bonding wire (the wire-bonding method). This method is effective for connecting the terminals located around the chip edge to the board. Recent developments in LSI logic, such as gate array or microcomputer logic, have led to increased chip density and improved function and led to an increased number of terminals per board. As a result, it is necessary that input-output terminals be located at the center of the chip surface. The conventional wire-bonding method for electrically connecting semiconductor chips to a mounting board cannot be adopted.
Alternately, another method (so-called the flip-chip method) is noted, wherein semiconductor chips are connected to a mounting board through bump terminals, which are solder bumps formed on the chips. In this method, the bumps on a chip are connected to the board on the back of a chip. This method allows for increased chip density since it is possible to form the terminals not only on the periphery of the chips, but also on the inner surface.
The bonding process with the flip-chip method is shown in FIGS. 1 through 5. At first (FIG. 1), the solder bumps 3 are formed on the terminals 2 of a semiconductor chip 1. The methods for forming solder bumps include solder vapor deposition, solder plating, and paste printing. In addition to the flip-chip method, the BGA (Ball Grid Array) method is available as a similar bonding method. BGAs are usually formed using either the solder ball supplying method or solder paste printing method. In either method, the bonding surface 4 of the solder bump 3 is then flattened as required. Then, the solder bumps 3 formed on the chip 1 are aligned with terminals 11 on the mounting board 10, as shown in FIG. 2. Flux 12 is then applied on the surface of terminals 11 on the mounting board 10, as shown in FIG. 3. One of functions of flux 12 is to temporarily fix the chip with its high viscosity. Flux 12 may also be applied to the solder bumps 3. As shown in FIG. 4, the solder bumps 3 come in contact with the terminals 11 on the mounting board 10 through the flux 12. The arrangement is fixed to prevent movement and the temperature is slowly raised to reflow the solder. It is important that the temperature be raised slowly; if heated rapidly, the chip might move and misalignment might occur before solder melting due to flux vaporization during the heating. After reflowing the solder, the terminals 2 on the semiconductor 1 are electrically connected to the terminals 11 on the mounting board 10 via the solder 3, as shown in FIG. 5. Upon cooling, the flux shrinks and adheres to the solder, shown in 15 in FIG. 5. Preferably, the flux will be removed during the subsequent cleaning process.
A common method for solder reflowing is to put the object in a hot-air furnace or in an infrared heating reflow furnace. In contrast, as disclosed in the Japanese Published Patent Application 6-124980, for example, another method has been proposed to provide an air-heating mechanism on a nozzle that grasps the chip at the time of its alignment and melts the solder with hot air.
In this process, since the forming method of the solder bumps 3 has an effect on the process cost, a low-cost method should be adopted. Conventionally, the solder vapor deposition and the solder plating method have been adopted as the technique for forming bumps for high-density patterns on a ceramic substrate, but the facility cost is high and productivity is not sufficient when using these methods. Consequently, the screen printing method is recommended as a low-cost method. In this method, a mask that only has holes for solder paste at solder bump locations is set on the chip. The solder paste is applied by squeezing the solder paste through the holes of the mask. The bumps are thus formed by transferring bulks of solder paste only at the required positions on the chip. Frequently the height of the solder deviates in this kind of low-cost method, and a flattening of the contact surface of the solder bumps is required (see FIG. 1).
Further, in these methods, pressure should be applied to solder the chips to a mounting board. When 324 bumps are formed on the present chip product, for example, approximately 650 gf pressure should be required, assuming that 2 gf is required per bump. In the future, as the number of bumps approaches to 2000, it is anticipated that the required pressure will be as large as approximately 4 Kgf. But applying such a large pressure on the chip might cause harmful influences on the chip function or on other aspects. Optimization of the process parameters to prevent the anticipated harmful influences is very complicated.
In summary, the prior art identifies the following items as problems in forming solder bumps and mounting chips:
(1) Flattening the bumps is necessary to eliminate solder height variation. PA1 (2) Appropriate pressure is required for mounting chips. PA1 (1) to provide a method wherein the solder bumps on the chip are contacted to the terminals on the mounting board via molten solder; and PA1 (2) to provide a method wherein no misalignment occurs during the soldering.
To solve these problems, a method is proposed in the Japanese Published Patent Application 4-83353, wherein both the chip and the mounting board are preheated during a prior contact between the terminals on the chip and those on the mounting board. Then the soldering is performed by continuously heating the back of the chip with radiant heat. A certain distance is maintained between the heating block and the back of the chip. This method can heat solder bumps equally by preheating the mounting board, and it does not squeeze the solder bumps because of a weakly applied pressure during the solder melting. However, there is a possibility of misalignment of the chip due to vaporization of the flux-volatile ingredients, since the chip is not held by the heating block during the solder melting.
In view of productivity, it is desirable to eliminate the step of flattening the solder bumps. The solder bumps must be in contact with the terminals of the mounting board during solder melting, not when the solder is solid. If the solder bumps on the chip contact the terminals on the mounting board when the solder is melted, pressure is not required during the chip mounting. Further a method must be provided to prevent misalignment because of vaporization of flux-volatile ingredients during the solder melting.
The problems that should be solved by this invention are: